Field of the Invention
The present invention relates to a gasket which is one kind of sealing devices, and more particularly to a seal bead structure of a gasket in which a through hole is provided in a flat surface portion of a metal gasket, and a seal bead is provided in a peripheral edge portion of the through hole. The gasket according to the present invention is used, for example, in a field relevant to an automobile, or is used in the other fields.
Description of the Conventional Art
Conventionally, for example, as a gasket which is used in a water pump relevant to the automobile, there has been known a gasket in which a plurality of through holes penetrating in a thickness direction are provided in a flat surface portion of a tabular metal gasket, and a seal bead is three-dimensionally molded in a peripheral edge portion of each of the through holes. In the gasket mentioned above, in the case that it is necessary to seal between the adjacent through holes, and it is impossible to set full beads in parallel since a distance (a seal width) between the through holes is narrow, the seal between the through holes 61 and 71 is carried out by employing the following structure.
(1) A structure in which two full beads 62 and 72 are connected as one full bead 82, as shown in FIG. 3; or
(2) a structure in which two half beads 63 and 73 are connected as one trapezoidal bead 83 (FIG. 4) or one full bead (FIG. 5) 84, as shown in FIG. 4 or 5.
In FIGS. 3 to 5, FIGS. 3A, 4A and 5A are plan views of the metal gasket in which a plurality of through holes 61 and 71 are provided in a flat portion 51 so as to pass through in a gasket thickness direction (a direction which is orthogonal to a paper surface in the drawings), and each of the beads is provided in a peripheral edge portion of each of the through holes 61 and 71. FIGS. 3B, 4B and 5B show schematic cross sectional views along a line C-C, a line D-D and a line E-E in the respective FIGS. 3A, 4A and 5A.
In features of three kinds of seal beads, a sealing performance (pressure) has the following relationship:
full bead>trapezoidal bead>half bead (FIG. 6), and
a bead compression property and followability have the following relationship:
half bead>trapezoidal bead>full bead (FIG. 7).
An appropriate bead is selectively used in correspondence to a used requirement (a required pressure or a housing rigidity). In FIGS. 6 and 7, solid lines on the graphs indicate the full bead, single-dot chain lines indicate the trapezoidal bead, and dotted lines indicate the half bead, respectively.
Among them, in the case that a working pressure range is high, a portion between the through holes 61 and 71 is sealed by the structure (1) mentioned above, however, since the bead rigidity becomes extremely higher in a combined portion (a peak portion) which connects two full beads 62 and 72 to one full bead 82, a surface pressure of its peripheral portion is lowered, and there is fear that the sealing performance is lowered.
As a countermeasure thereof, in a seal bead structure described in Japanese Unexamined Patent Publication No. 2008-157377, a flat portion is provided by forming a combined portion 85 as a three-point connection in place of a pin point structure, and a surface pressure concentration is prevented in the combined portion 85 by setting a crossing angle to 25 to 45 degrees. As a result, uniformization of the bead surface pressure is achieved between the portion constituted by one bead and the portion constituted by two beads.
In the case that a bolt pitch is long, in the case that a thickness of an opponent housing is small, and in the case that the through holes 61 and 71 which are away from a bolt are sealed, the opponent housing is significantly deformed since the full beads 62 and 72 have greater generated reaction force on the assumption that an amount of compression is fixed as shown in FIG. 7. As a result, it is impossible to sufficiently compress the gasket, and the full beads 62 and 72 can not be applied.
Therefore, the case mentioned above is dealt with the structure (2) in which the half beads 63 and 73 having good compression property are combined with the trapezoidal bead 83 or the full bead 84. However, since the reaction force is not generated in the half beads 63 and 73 until the half beads 63 and 73 are compressed to about 50% of the bead height while the reaction force is generated in the trapezoidal bead 83 and the full bead 84 just after starting the compression, it is necessary to set a compression restoring property of the trapezoidal bead 83 and the full bead 84 so that the half beads 63 and 73 can be compressed 50% or more in the vicinity of the combined portion (the peak portion) 85. Accordingly, the reaction force is specifically made smaller by setting the bead width of the trapezoidal bead 83 or the full bead 84 to be wider, however, this structure can not deal with the case that the seal width between the through holes 61 and 71 is narrow.
As the other prior arts in relation to the present invention, there are seal bead structures which are described in Japanese Unexamined Patent Publication No. 8-121596 and Japanese Unexamined Patent Publication No. 9-229195, however, these seal bead structures do not form a common bead portion provided between a plurality of through holes in a special shape.